/Engine/Plugins/Runtime/XRBase/Source/XRBase/Public/HeadMountedDisplayBase.h
Implement this method to override the render target for scene based stereo layers. Return nullptr to render stereo layers into the normal render target passed to the stereo layers scene view extension, in which case OutViewport must not be modified.
virtual FTexture2DRHIRef GetSceneLayerTarget_RenderThread ( int32 ViewIndex, FIntRect & InOutViewport )